Solder bumps are used to solder electronic parts to printed circuit boards. For example, with semiconductor packages such as BGA (ball grid array) devices and a considerable proportion of CSP's (chip size packages) and MCM's (multi-chip modules), solder bumps are formed on the electrode pads of a substrate for the package and used to electrically and physically connect the package to a printed circuit board by reflow soldering.
Solder bumps are also used as means for wireless bonding of a semiconductor chip to a substrate or to a printed circuit board. Wireless bonding methods using solder bumps include the TAB (tape automated bonding) method and the flip chip method. The flip chip method is one form of the DCA (direct chip attachment) method (also called the bare chip assembling method). Although the DCA method can also be performed by wire bonding, wireless bonding using solder bumps, such as the flip chip method, has recently been increasingly employed in the DCA method, particularly in the field of mobile or portable electronic products.
Wire bonding technology is extremely advanced, but it still requires a certain amount of time, since the electrodes of an electronic part must be bonded in sequence rather than simultaneously when wire bonding is employed, and the gold wires which are used for wire bonding are expensive. Furthermore, when a semiconductor chip has a large number of electrodes near its center, it may be impossible to form connections to all the electrodes by wire bonding, since some of the wires may end up contacting each other, which is not permissible.
These problems of wire bonding are eliminated by wireless bonding. Wireless bonding is in general less expensive than wire bonding because gold wires are no longer used, and its productivity is superior since a large number of connections can be formed at the same time. In addition, the problem of wires contacting each other does not occur.
Solder alloys which have conventionally been used for soldering of electronic parts have been, in most cases, Pb—Sn solder alloys. Pb—Sn alloys have excellent solderability and relatively low melting temperatures, so they are very suitable for soldering electronic parts to printed circuit boards with little occurrence of soldering defects, thus making it possible to form reliable soldered joints.
When electronic equipment which has been manufactured using a Pb—Sn solder alloy malfunctions or becomes obsolete, it is frequently simply discarded instead of being repaired or renovated. Some of the constituent materials of discarded electronic equipment (metal used in frames, plastic used in cases, glass used in displays) can be recovered and reused, but printed circuit boards in discarded electronic equipment generally cannot be reused, so they are typically disposed of by burial in landfills. This is because the resins and copper foil used to form a printed circuit board are tightly adhered to each other, and solder is metallically bonded to the copper foil, so these materials cannot be readily separated from one another.
If printed circuit boards which are disposed of by burial are contacted by acid rain which has seeped into the ground, Pb in the solder is dissolved out of the solder by the acid rain, and water containing Pb seeps further into the ground and mixes with underground water and may enter the water supply. If water containing Pb is consumed for long periods by humans, the Pb accumulates within the body and may eventually cause lead poisoning. To avoid the harmful effects of Pb in water supplies, the use of Pb in solder is now regulated around the world, and as a result, there is a great interest in the electronics industry in lead-free solders which do not contain Pb.
Most lead-free solders are based on Sn, and further include one or more alloying elements such as Ag, Cu, Bi, In, Zn, Ni, Cr, P, Ge, and Ga.
Some examples of binary lead-free solder alloys which have been used in the past are Sn—Cu, Sn—Sb, Sn—Bi, Sn—Zn, and Sn—Ag solder alloys. In general, Sn-based lead-free solders have inferior solderability compared to conventional Pb—Sn solders. Particularly, Sn—Cu and Sn—Sb solder alloys have poor solderability. Sn—Bi solder alloys tend to form brittle soldered joints, which are not only readily peeled off upon application of an impact, but may be lifted off if they are contaminated with a small amount of Pb which is introduced into the joints from the plating on a lead. Sn—Zn solder alloys have problems because Zn is a base metal. For example, when a Sn—Zn solder alloy is used in the form of a solder paste, the paste may degrade rapidly to such a degree that it cannot be applied by printing, or galvanic corrosion may occur in soldered portions after soldering is completed. Thus, among Sn-based lead-free solders, Sn—Ag solder alloys are superior to other binary lead-free solder alloys with respect to properties such as solderability, brittleness, and aging (storage stability).
However, Sn—Ag lead-free solder alloys do not always have sufficient bonding strength, particularly when used to form joints having a very small bonding area. There is an ongoing trend to increase the performance while decreasing the size of electronic equipment, which creates a demand for increases in the performance and decreases in the size of electronic parts incorporated into such equipment. Taking a BGA device as an example, the number of electrodes formed thereon is increasing while the overall size thereof is decreasing. As a result, the most compact BGA devices have nearly the same tiny pitch between solder bumps as CSP's (chip size packages), so they are also considered to be CSP's. Thus, the diameters of solder bumps to be formed on the electrode pads of electronic parts are also becoming smaller, but at the same time, a bonding strength comparable to that of larger-diameter bumps is demanded. While the bonding strength of a conventional Sn—Ag lead-free solder alloy may be sufficient when it is used to form large-diameter bumps, the bonding strength is not adequate for small-diameter solder bumps such as those formed on CSP's.
During manufacture, an electronic part is often subjected to a burn-in test, which determines whether the electronic part is capable of operating properly at an elevated temperature. The conditions of a burn-in test differ depending upon the manufacturer, but typical test conditions are a test period of 12 hours at a temperature of 125° C. After a burn-in test, if the electronic part is one having solder bumps, the part is inspected with image processing equipment to determine whether any of the solder bumps are missing or misshapen.
It has been observed that the surface of a solder bump formed from a Sn—Ag lead-free solder alloy on an electronic part may become yellowed during a burn-in test. If the surface of a solder bump becomes yellowed, image processing equipment used in subsequent inspection of the solder bump may not accurately detect solder bumps which have been yellowed, thereby producing inaccurate results. For example, there is the possibility of defective solder bumps in a part not being detected or of a part having satisfactory bumps being erroneously rejected as defective. Thus, yellowing of solder bumps during burn-in testing is undesirable.